meson: Build only C API

Hi,

I'm trying to use the WirePlumber C API in a flatpak, and since it is not included in the freedesktop runtime, I need to build it myself.

Unfortunately, there seems to be no option in meson_options.txt to avoid building the daemon, and I haven't managed to find a good way to build and install only the "wireplumber-0.4" target.
Obviously compiling and shipping the entire daemon in a flatpak just to use the C API would be a waste.

Could we add an option (true by default) that disables building the daemon, the lua dependency, etc. when set to false?

Thanks!